Speakers have relatively low efficiency in the conversion of an electrical audio input signal into mechanical or acoustical output (sound waves). Most of the input energy is used to heat a voice coil that moves a diaphragm to produce the sound waves. Although some materials may operate at relatively high temperatures, including certain permanent magnet materials that are used in the magnet system of the speaker, excessive temperature can result in damaging the speaker. In addition, certain characteristics of audio signals can also lead to increased or even excessive temperature in a speaker. When a speaker is producing a high volume of sound for an extended amount of time, the amount of power being dissipated may rise to a sufficiently high level that causes the speaker to rise to very high temperatures thereby putting the speaker at risk for damage by overheating. Typically, the risk of heat damage rises when continuously high sound levels are being produced, for a fairly long period of time, rather than in response to sharp spikes. It is possible to limit the amplitude of the audio signal that is driving the speaker, namely by attenuating the signal (or reducing the gain applied to it), based on, for instance, the speaker's nominal power rating and impedance. A simple RMS voltage limiter, however, neglects the fact that a speaker can usually handle large RMS voltages for sufficiently short periods of time, so that approach is likely to provide too much limiting. Another approach is to monitor the voltage and current that is being delivered by the power amplifier to the speaker. In yet another solution, a detailed thermal model of a speaker is defined, and is then used to continuously calculate an estimate of the temperature of, for instance, the speaker voice coil, as the input audio signal is driving the voice coil.
The thermal model approach may track the voltage that is being applied to the terminals of a speaker, and then uses the measured voltage to calculate or predict the instantaneous temperature of, for instance, the voice coil. As the estimated temperature varies and crosses predefined thresholds, a thermal gain control algorithm responds by continuously varying the gain (attenuation) that is applied to the audio signal (that drives the speaker), in order to prevent the speaker from overheating. The thermal model uses many input parameters to compute the estimated temperature.